1. Field of the Invention
This invention relates to analog gauges and specifically to flat response temperature circuitry for analog gauges yielding a decreased parts count and increased control over a xe2x80x9cflat zonexe2x80x9d of the gauge.
2. Description of Related Art
Analog gauges are commonly used to display automobile data to a driver. In a typical analog gauge, electrical current flows through wire coils disposed about a permanent magnet. The amount of electrical current flowing through each coil varies according to the value of a measurand at a remote location.
As current flows through each coil, a magnetic field B is induced proximal to the coil. The direction of the magnetic field is determined by the direction of the winding of the coil as given by the right-hand-rule. In general, a stronger magnetic field can be created by allowing more current to pass through a given coil. The strength and direction of the magnetic field can be represented by a vector having a magnitude corresponding to the strength of the magnetic field and a direction corresponding to the direction of the induced magnetic field.
The magnetic fields induced about each coil combine to create a resultant magnetic force which is, in terms of direction, followed by the permanent magnet about which the coils are disposed. The permanent magnet is attached to a rotatable shaft that is attached to a pointer arm that moves over a dial face in response to changes in the direction of the resultant force. Circuitry, attached to the coils, varies the relative current flow in each coil to change the resultant magnetic vector corresponding to the value of the measurand at the remote location. If the measurand changes, the direction of the resultant force will change and the shaft and pointer will rotate accordingly.
In a linear gauge, the shaft responds in a linear relationship to changes in the measurand at the remote location. For example, in a linear temperature gauge, a 20% change in temperature causes a 20% rotation in the magnet, shaft, and pointer. Alternatively, the responsiveness of the gauge can be reduced for a predetermined range of temperatures. Such a gauge is commonly referred to as a xe2x80x9cflat responsexe2x80x9d gauge because a xe2x80x9cflat zonexe2x80x9d is created in which the circuitry of the gauge has a reduced level of responsiveness to changes in the measurand.
The prior art circuit of FIG. 1A is exemplary and provides for a power source 10 such as a DC battery and a bridge resistor 12 having one terminal connected to the power source 10 and one terminal connected to a sender resistor 14 such as a thermistor. The sender resistor 14 has its remaining terminal connected to ground. This sender resistor 14 has an operating resistance of 275xe2x88x9218.3 xcexa9.
Further connected to the power source 10 is a first coil L1 having one terminal connected to the power source 10 and one terminal connected to a second coil L2. L2 is in series with L1 and has its remaining terminal connected to a third coil L3. L3 has one terminal connected to L2 and one terminal connected to a fourth coil L4. L4 has its remaining terminal connected to an anode of first diode 16 whose cathode 16C is connected to ground.
L3, L4, and the first diode 16 are connected in series, and L1 is wound about a first axis, L2 and L3 are counterwound about the same first axis, and L4 is counterwound about a second axis which is magnetically orthogonal to the first axis. L1 and L2 are formed from a single piece of uninterrupted wire having a resistance of 235.2 xcexa9, and L3 and L4 are formed from a single piece of uninterrupted wire having a resistance of 100.6 xcexa9. L1 comprises 1290 turns of wire; L2, 490 turns; L3, 630 turns; and L4, 310 turns.
The prior art circuit further includes a zener diode 18 connected at its anode 18A to the common terminal between L2 and L3 and at its cathode 18C to the common terminal between the bridge resistor 12 and the sender resistor 14. The zener diode 18 is a 3.6 V, 1 W zener diode, and, dependent on the resistance of the sender resistor 14, it provides a current path when reverse biased or forward biased, as will be elaborated upon below. The zener diode 18, in conjunction with the resistance of the sender resistor 14, establishes the flat zone responsiveness of the circuit.
Referring now to FIG. 1B, the magnetic fields induced by the electrical currents flowing through each coil L1-L4 are depicted by individual vectors B1-B4, respectively, each vector having a magnitude corresponding to the strength of the related magnetic field and a direction corresponding to the direction of the related magnetic field according to the right hand rule oriented along the appropriate winding axis. Because coils L1, L2, and L3 are wound about the same magnetic axis, their respective magnetic fields, B1, B2, and B3, lie along a common axis. Stronger magnetic fields are represented by vectors having greater magnitudes along the appropriate axes, and the direction of the magnetic fields induced by coils L2 and L3 (i.e., B2 and B3, respectively) are aligned with one another because both are wound about the same axis in the same direction, as opposed to the magnetic field induced by coil L1 (i.e., B1), which is counterwound about the same magnetic axis in the opposite direction. The magnetic field induced by coil L1 therefore magnetically opposes the fields induced by coils L2 and L3. The magnetic field induced by coil L4 (i.e., B4) is magnetically orthogonal to the magnetic field induced by coils L1-L3 because L4 is wound about a second axis which is magnetically orthogonal to the first. What is needed, however, is circuitry having magnetic fields induced in all four directions from the common origin located at the intersection of the winding axes of the coils L1-L4.
Finally, a resultant magnetic force acting on the permanent magnet can be represented by a resultant vector B having a magnitude and direction which is equal to the sum of the individual magnitudes and directions of the magnetic fields B1-B4 induced by the coils L1-L4, respectively. The direction of the resultant vector corresponds to the direction of the resultant force and determines the amount of rotation of the permanent magnet, shaft, and pointer, which are fixedly attached to one another.
Unfortunately, however, traditional flat response circuitry has significant drawbacks. For example, a diode must be connected in series between ground and L4. That is, the coil that is furthest from the power source, in order to provide a voltage drop to allow adjusting the flat zone responsiveness of the circuit. Moreover, different manufacturers require different flat response curves for arbitrary sender resistances, and the circuitry of the prior art does not allow the flexibility required to implement different flat response curves.
What is needed, therefore, is circuitry allowing increased control over the flat zone responsiveness of a non-linear gauge. Such circuitry must be flexible enough to meet the demands of numerous manufacturers utilizing different sender resistors and demanding differing levels of angular displacements of the pointer arm over the dial face.
Briefly, the circuitry of the present invention comprises a plurality of coils wound about a first axis and a plurality of coils wound about a second axis, the second axis being magnetically orthogonal to the first axis. A single zener diode is provided having its cathode connected to a common terminal between a bridge resistor and a sender resistor and its anode connected to a common terminal between the coils wound about the second axis. In an embodiment described below, the sender resistor is a thermistor and the zener diode conducts in a forward or reverse direction dependent upon the resistance of the sender resistor compared to the bridge resistor. This circuitry provides a flat zone responsiveness without a second diode. By eliminating the need for this second diode, cost and circuit space are saved and the reliability of the gauge is increased. The circuitry is simplified because the number of turns of each coil and hence the relative strength of the four magnetic fields can be readily adapted to operate with diverse senders. Thus, it is no longer necessary to individually adjust and calibrate the sender resistance based upon specified levels of flat zone responsiveness.
Accordingly, it is an object of the present invention to provide non-linear gauge circuitry having increased flexibility with respect to setting the flat zone responsiveness of the circuit. It is a further object of this invention to achieve this increased flexibility with a minimum number of circuit elements. It is another object of this invention to provide circuitry having magnetic fields induced in four directions from an origin located at the intersection of the winding axes of the coils. It is still another object to provide circuitry yielding increased control over the resultant force comprised of the summation of the individual magnetic fields induced about the individual coils of the circuit.
The foregoing and other objects, advantages, and aspects of the present invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown, by way of illustration, a preferred embodiment of the present invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must also be made to the claims herein for properly interpreting the scope of this invention.